The present invention relates to a method and a system for charging one or more supercapacitors, more particularly suitable for balancing supercapacitors.
Supercapacitors are known in the art and are currently being developed as power sources in high-power applications such as engine starting, power top-up for hybrid vehicle motors, and uninterruptible power supplies. In applications like these, power supplies are needed that can be charged quickly and can perform a very large number of cycles, which is the case with supercapacitors, but not with traditional batteries.
Supercapacitors are capable of delivering very high specific powers over short time periods. The characteristic discharging (or charging) time of a supercapacitor is of the order of a few seconds to a few tens of seconds, during which time specific powers exceeding 1 kW/kg can be delivered. Individual supercapacitors have a capacitance from 1 F to approximately 3,500 F and a very low resistance, of less than 1 mxcexa9 for components with the highest capacity.
When charging supercapacitors, it is important not to exceed a maximum voltage at the terminals of the supercapacitor. Controlling the charging of a supercapacitor so that charging is stopped if the voltage at the terminals reaches a predetermined value is known in the art. If the voltage exceeds this predetermined value, aging of the supercapacitor is accelerated, which reduces its autonomy and power.
A supercapacitor module generally comprises a plurality of supercapacitors connected in series. The applications mentioned above generally require voltages exceeding a few tens of volts, or even a few hundreds of volts. In this case, at the end of charging the supercapacitor module, a spread of the characteristics of the supercapacitors relative to each other is observed, and this applies in particular to the voltage at the terminals of the supercapacitors. This is due to a spread of the intrinsic properties (series resistance and capacitance) of each supercapacitor in the module, to aging of the supercapacitors, and possibly to a temperature gradient within the module, due to its environment. This leads to different leakage currents for each of the supercapacitors of the module and therefore to different end of charging voltages for each of the supercapacitors.
This problem compromises correct operation of the supercapacitor module. Some supercapacitors of the module may reach voltages exceeding their nominal charging voltage, which degrades their characteristics and leads to premature aging. Thus the module as a whole cannot function correctly.
To solve this problem, the document EP-0 564 149 proposes to connect a bypass circuit in parallel with the terminals of each supercapacitor of a module comprising several supercapacitors, the bypass circuit comprising an MOS transistor that changes state at a predefined voltage at the terminals of the supercapacitor to bypass a nominal bypass current.
However, this causes problems. The bypass circuit implies the use of a charging current equal to the nominal bypass current at the end of charging. If the charging of a supercapacitor continues at a current much higher than the nominal bypass current, the supercapacitor may be overcharged, reducing its service life. Consequently, using a low charging current equal to the nominal bypass current will very greatly increase the duration of the end of charging period.
The present invention aims to provide a supercapacitor charging method enabling charging of the supercapacitor to continue at a current higher than the nominal bypass current after the bypass circuit has begun to bypass the current, thereby reducing the duration of the end of charging period.
The remainder of the description refers to balancing in relation to a supercapacitor module and a supercapacitor in isolation, in which case it is more a question of monitoring the charging voltage of the supercapacitor.
To this end the present invention proposes a method of charging at least one supercapacitor including a step of bypassing the current flowing in said supercapacitor so that, when the voltage at the terminals of said supercapacitor reaches a predetermined value, constituting a threshold voltage, the bypass current assumes a maximum value constituting a nominal bypass current, which method is characterized in that it includes a step of monitoring of the charging current of said supercapacitor as a function of the voltage at the terminals of said supercapacitor by a voltage detector logic function, constituting an optimization function, able to change from an activated state to a deactivated state when the voltage at the terminals of said supercapacitor exceeds a first predefined voltage greater than the threshold voltage and then to return to the activated state when said voltage at the terminals of said supercapacitor falls below a second predefined voltage less than or equal to said first predefined voltage.
Thanks to the invention, charging of the supercapacitor can continue at a current greater than the nominal bypass current after bypassing has started, with the charging current monitored as a function of the voltage at the terminals of the supercapacitor to eliminate the risk of exceeding a predefined voltage that can reduce the service life of the supercapacitor. The method according to the invention therefore optimizes the charging time without damaging the supercapacitor.
The signal delivered by said optimization function is advantageously a hysteresis signal.
One embodiment of the method includes a step of charging said supercapacitor with a charging current greater than said nominal bypass current and maintaining said charging step for as long as said optimization logic function is in the activated state.
Advantageously, when the voltage at the terminals of said supercapacitor returns to a voltage value, constituting a reference voltage, which is less than the value of said threshold voltage, the bypass current assumes a value very much less than a current corresponding to the leakage current of said supercapacitor.
In a different embodiment, the characteristic of the bypass current as a function of the voltage at the terminals at said supercapacitor is a hysteresis signal.
The binary nature of the bypass current as a function of the voltage at the terminals of the supercapacitor thus has a hysteresis shape and improves stability in the event of variation in the voltage of the supercapacitor, for example due to a sudden variation in the charging current.
In one embodiment, the method includes a step of filtering high-frequency harmonics of said voltage at the terminals of said supercapacitor.
Thus using a low-pass filter filters out high-frequency harmonics of the voltage at the terminals of the supercapacitor induced by high-frequency switching of the charging current.
One embodiment of the method includes a step of reinitializing of the charger as a function of the voltage at the terminals of the supercapacitor by a voltage detector logic function, constituting a reinitialization function, able to change from an activated state to a deactivated state when the voltage at the terminals of said supercapacitor exceeds a third predefined voltage less than or equal to said first predefined voltage and then to return to said activated state when said voltage at the terminals of said supercapacitor falls below a fourth predefined voltage less than said second predefined voltage.
The method advantageously includes a step of detection of a minimum safety voltage at the terminals of the supercapacitor by a voltage detector logic function, constituting a minimum voltage detector function, able to change from an activated state to a deactivated state when the voltage at the terminals of said supercapacitor exceeds a fifth predefined voltage and to return to the activated state when said voltage at the terminals of said supercapacitor falls below a sixth predefined voltage.
Thus if the voltage at the terminals of the supercapacitor becomes very low (discharging), the detector function reduces the discharge current.
The charging method advantageously includes a step of charging a plurality of supercapacitors, characterized in that said optimization function changes from an activated state to a deactivated state when at least one of the voltages at the terminals of said supercapacitors exceeds said first predefined voltage and returns to the activated state when each of the voltages at the terminals of said supercapacitors falls below said second predefined voltage.
The charging method advantageously including a step of charging a plurality of supercapacitors is characterized in that said reinitialization function changes from an activated state to a deactivated state when at least one of the voltages at the terminals of said supercapacitors exceeds said third predefined voltage and returns to the activated state when each of the voltages at the terminals of said supercapacitors falls below said fourth predefined voltage.
The charging method advantageously including a step of charging a plurality of supercapacitors is characterized in that said minimum voltage detector function changes from an activated state to a deactivated state when each of the voltages at the terminals of said supercapacitors exceeds said fifth predefined voltage and returns to the activated state when at least one of the voltages at the terminals of said supercopacitors falls below said sixth predefined voltage.
The charging method advantageously including a step of charging a plurality of supercapacitors includes a step of detection of dispersion of the voltage at the terminals of said supercapacitors by a voltage detector logic function, constituting a dispersion detector function, characterized in that said dispersion detector function changes from an activated state to a deactivated state when each of the voltages at the terminals of said supercapacitors exceeds a seventh predefined voltage and returns to the activated state when at least one of the voltages at the terminals of said supercapacitors falls below an eighth predefined voltage.
The combination of the end of charge optimization function and this dispersion detector function detects significant imbalance of the voltages at the terminals of the supercapacitors in a supercapacitor module.
Finally, the present invention also provides a system for implementing the method according to any preceding claim, which system comprises:
at least one supercapacitor,
a bypass circuit comprising a transistor operating in switching mode and connected in parallel with the terminals of said supercapacitor,
charging means, and
a detector unit delivering at least one logic signal representing the voltage at the terminals of said supercapacitor, said logic signal being supplied to said charging means.
The system advantageously comprises a low-pass filter connected in parallel with the terminals of said supercapacitor.
A first embodiment of the system includes a plurality of supercapacitors connected in series, a bypass circuit being connected in parallel to the terminals of each of said supercapacitors.
A second embodiment of the system includes a plurality of supercapacitors connected in series, a single bypass circuit being connected in parallel to the terminals of all of said supercapacitors.
A third embodiment of the system includes a plurality of supercapacitors connected in parallel, a single bypass circuit being connected in parallel to the terminals of all of said supercapacitors.
Other features and advantages of the present invention will become apparent in the course of the following description of one embodiment of the invention, which is given by way of illustrative and non-limiting example.